The present invention relates to an optical wave-length filter and a driving method thereof.
An optical wave-length filter which can vary and adjust a wave length of transmitted light has been proposed in recent years.
This kind of conventional type filters is, for example, disclosed in Literature I entitled: "Guided-Wave Optoelectronics, p. 193.about.197, by Springer-Verlag Co. Ltd., 1988", which is constituted by a single mode-converter which can vary a transmitted wave-length, that is, a predetermined standard wave-length .lambda..sub.O, within a range of adjustable deviation wave-length .DELTA..lambda..sub.T.
In this filter, the adjustable deviation wave-length .DELTA..lambda..sub.T can be described as .DELTA..lambda..sub.T =.LAMBDA..multidot..DELTA.Ne, where .LAMBDA. is a period of comb-type electrodes and .DELTA.Ne is a refractive index deviation of a wave-guide route in case the refractive index of the wave-guide route of the optical wave-length filter being electrically varied.
The conventional optical wave-length filter disclosed above has, however, a shortcoming that the maximum value (=.DELTA..lambda..sub.T (max)) of the adjustable deviation wave-length .DELTA..lambda..sub.T is so small that the adjustable range of the transmitted wave-length of the optical wave-length filter is restricted to be narrow.
In this filter, assuming that the standard wave-length .lambda..sub.O is 1.3 .mu.m and Ti diffused wave-guide route is formed in LiNbO.sub.3 substrate, .LAMBDA. will be approximately 18 .mu.m and .DELTA.Ne in a range of 10.sup.-3 so that an upper limit of .DELTA..lambda..sub.T (max) will be approximately 20 .mu.m.
Since .DELTA..lambda..sub.T (max).perspectiveto.20 .mu.m does not comply with a practically sufficient variable range of the transmitted wave-length, another alternative structure illustrated in FIG. 1 has been proposed to expand the adjustable range of the transmitted wave-length.
FIG. 1 is a perspective view illustrating one example of a multi-stage conventional optical wave-length filter.
The optical wave-length filter illustrated in the drawing is constructed such that a wave-guide route 12 is formed in a substrate 10, that polarizers 14 and 16 are respectively disposed at both sides of an input port 12a and an output port 12b of the wave-guide route 12, and that three (3) conventional filters (hereinafter called as mode-converter) 18 are, for example, disposed between these polarizers.
In this filter, a wave-guide type polarizer 14 permits to transmit TE mode light therethrough, a wave-guide type polarizer 16 permits to transmit TM mode light, and a mode-converter 18 converts the TE mode light having a predetermined transmitted wave length into the TM mode light.
As the result, light having a predetermined transmitted wave-length can be selected separately among lights have a plurality of different wave lengths.
This optical wave-length filter is designed such that the predetermined standard wave length .lambda..sub.O of the transmitted wave-length of each mode-converter 18 is set, departing each other, with an approximately .DELTA..lambda..sub.T (max) distance.
The mode-converter 18 is constituted by a pair of comb-type electrodes interdigitally coupled each other and finger-type electrodes 18a of the comb-type electrodes are disposed a certain distance from each other along wave-guide route 12.
Since the period of disposed position of electrodes 18a of the comb-type electrode is set to differ each other in each mode-converter 18, the standard wave-length .lambda..sub.O of the transmitted wave-length also differs from each other in each mode-converter 18.
When the voltage V1 is applied in a similar manner as in a conventional filter to the comb-type electrodes of the mode-converter 18, each of which is corresponding to the predetermined transmitted wave-length, optical mode conversion can be performed with a predetermined transmitted wave-length.
The band width .DELTA..lambda..sub.W of the transmitted wave-length can be described as; EQU .DELTA..lambda..sub.W /.lambda.=.LAMBDA./L,
wherein .lambda. is a wave length transmitted through the optical wave-length filter, and L is a distance (length of mode-converter) between the first finger-type electrode 18a and the last finger-type electrode 18a of mode-converter 18.
Accordingly, in order to shorten the band width .DELTA..lambda..sub.W for improving filter characteristics, the length L of the mode-converter is needed to be large.
The conventional filter, however, necessitates n unit mode-converter 18 being coupled in series to select light having a predetermined wave-length among different lights having n wave-lengths, each of which exists departing each other in distance of .DELTA..lambda..sub.T (max).
As the result, there is a shortcoming that the total length of the optical wave-length filter requires, at least, length of nL.